The present invention relates to a method of chemical decontamination. In more particular, the present invention relates to a method of chemical decontamination which is suitable for an application to boiling water reactor plants (BWR plants) using a boiling water reactor (BWR) and is used for removing radionuclides from surfaces of metallic members of a component and piping of a primary cooling system and a system comprising these which have been contaminated with radionuclides.
A known method used for chemically removing radionuclides from surfaces of a component and piping of a primary cooling system of a nuclear power plant (NPP) which contact with a coolant and which have been contaminated with the radionuclides is a method of chemical decontamination using alternately an oxidation decontaminating agent and a reduction decontaminating agent. The radionuclides are incorporated into oxides, which are present on surfaces of the component and piping, such as oxides containing much iron (hereinafter referred to as iron-based oxides), e.g. hematite (xcex1-Fe2O3), nickel ferrite (NiFe2O4) and magnetite (Fe3O4), and oxides containing much chromium (hereinafter referred to as chromium-based oxides), e.g. chromium oxide (Cr2O3) and iron chromite (FeCr2O4).
The iron-based oxides are readily soluble in acids and reducing agents, and the chromium-based oxides are readily soluble in oxidizing agents. In the method of chemical decontamination, accordingly, in order to remove the iron-based oxides and the chromium-based oxides which are present on the surfaces of piping and components, an oxidation decontaminating agent and a reduction decontaminating agent are alternately used.
A previously known method of chemical decontamination which uses an oxidation decontaminating agent and a reduction decontaminating agent alternately includes a method which chemically decontaminates metallic structural members of a reactor by using permanganic acid as the oxidation decontaminating agent and a dicarboxylic acid, such as oxalic acid, as the reduction decontaminating agent. This method is disclosed in JP-B-3-10919.
Japanese National Publication (Kohyo) No. 2-503600 discloses a method of chemical decontamination applied to a pressurized water reactor. In this method of chemical decontamination, first an oxidation decontamination using an oxidation decontaminating agent containing permanganic acid and chromic acid is conducted and then a reduction decontamination using a reduction decontaminating agent containing oxalic acid is conducted. The Publication discloses also that surface layers which have been changed by the oxidation decontamination, of materials generally used in a nuclear reactor, such as carbon steel, chromium-based stainless steel, nickel alloy and others, are completely removed by the reduction decontamination.
When oxidation decontamination and reduction decontamination are applied to a boiling water reactor plant to decontaminate a component and piping of a primary cooling system contaminated by radionuclides, a reduction decontamination using a reduction decontaminating agent is first conducted and an oxidation decontamination using an oxidation decontaminating agent is performed thereafter. This is because an amount of iron chromite to be dissolved by the oxidation decontaminating agent is small and iron oxides, such as hematite, to be dissolved by the reduction decontaminating agent are present in a large amount.
A boiling water reactor plant comprises structural members manufactured with stainless steel and structural members manufactured with carbon steel. Carbon steel is more readily dissolved by a reduction decontaminating agent, e.g., an oxalic acid solution, than stainless steel.
Magnetite of an iron-based oxide, which is formed much on a surface of a structural member in hot water, also dissolves more readily in oxalic acid solution than iron-based oxides such as hematite and nickel ferrite. In a boiling water reactor plant, therefore, chemical decontamination is presently applied only to parts of structural members manufactured with stainless steel.
The object of the present invention is to provide a method of chemical decontamination which can suppress a decrease of thickness due to corrosion of structural members in a nuclear power plant and can attain a removal of radionuclides with good efficiency.
To achieve the above-mentioned object, the method of the present invention comprises, in a nuclear power plant provided with a first structural member having a surface which contacts with a coolant and is made of stainless steel and a second structural member having a surface which contacts with a coolant and is made of carbon steel or an iron-based alloy containing chromium and being inferior in corrosion resistance to the stainless steel, first supplying an oxidation decontaminating solution containing an oxidation decontaminating agent into the first structural member and the second structural member and thereafter supplying a reduction decontaminating solution containing a reduction decontaminating agent into the first structural member and the second structural member.
Since an oxidation decontamination using an oxidation decontaminating solution is conducted first, a magnetite in an oxide film formed on a surface of the structural member changes into hematite, which is difficult to be dissolved by a reduction decontaminating solution. Consequently, even when a reduction decontamination using a reduction decontaminating solution is conducted after the oxidation decontamination, a decrease of thickness of the structural member due to corrosion is reduced. Moreover, since the decontamination of the first structural member and that of the second structural member can be conducted in parallel, the removal of radionuclides can be achieved with good efficiency even when structural member parts formed of different kinds of materials are the objects of decontamination.